1. Field of the Invention
The invention concerns an optical viewing device, such as for example a surgical (stereo) microscope, having an apparatus for partial reduction of the illumination intensity, such as an eclipse filter. An “eclipse filter” is understood to be a spectral filter in an illuminating beam which fully or partially reduces the illumination intensity in a specific light wavelength range.
2. Description of the Related Art
Reduction of the illumination of a specific spatial region of a specimen, by means of a stop, is more frequently being used in a variety of applications so that light sensitive surfaces, of the specimen can be protected from excessively strong radiation. These stops are generally configured to swing into and out of an illuminating beam thereby providing complete and/or partial darkening of the illuminating beam in a specific spatial region which is to be protected from the light.
German Patent Document DE 93 01 448 U, which is incorporated herein by reference, discloses a filter system. The filter system, which includes individual filters possessing different diameters, provides for a partial darkening in specific spatial regions. This darkening is not wavelength-dependent in the visible light region between 400 and 700 nm. In addition, German Patent Document DE 88 08 871 U, which is also incorporated herein by reference, provides a diffusion plate for slit lamp devices which plate creates a preferred direction in the scattering characteristic. Further, U.S. Pat. No. 4,715,704, which is also incorporated herein by reference, describes a light trap having a centrally darkened region.
Although the aforementioned patents have improved the art, various disadvantages remain. First, the reduction in light intensity is accomplished either in binary fashion (i.e., the reduced-light specimen region either appears dark to the user or is visible at full brightness), or there is a graduated filter effect which is not optimized in terms of light color; this can have negative effects on a specimen, for example on a patient's eye. Second, to make the reduced-light region visible in true color to the viewer, it is necessary to remove the stop thereby completely illuminating the specimen; such illumination may damage the specimen. Third, because of the fixed positioning of the stop in the illuminating beam, the shape and/or the attenuated illuminating beam cannot be varied. Fourth, damage to the specimen, for example to the patient's eye, usually does not occur in a similarly hazardous fashion over the entire range of light wavelengths. Rather, damage occurs only in a specific wavelength range, for example from 420 nm to 470 nm (or even up to 495 nm). In addition, below 400 nm, it is unnecessary to filter out all wavelengths.
In light of the aforementioned deficiencies in the prior art, a new apparatus is needed for reducing the intensity of light which is directed toward sensitive regions of a specimen which may be injured by excessively strong radiation.